The present invention relates to a wet etching method for MRAM magnetic tunnel junctions containing oxide based tunneling barriers.
MRAM is based on magnetic storage elements integrated with CMOS processing. Each storage element uses a magnetic tunnel junction (MTJ) device. The MTJ is generally composed of a fixed magnetic bottom layer (i.e., reference layer), a thin dielectric tunnel barrier layer, and a free magnetic top layer (i.e., freelayer), wherein the thin dielectric tunnel barrier layer is typically alumina (AlOx) or magnesium oxide (MgO). When a bias is applied to the MTJ, electrons that are spin polarized by the magnetic layers traverse the dielectric barrier through a process known as tunneling. The MTJ device has a low resistance when the magnetic moment of the freelayer is parallel to the fixed layer and a high resistance when the freelayer moment is oriented anti-parallel to the fixed layer moment. This change in resistance within the magnetic state of the device provides magnetoresistance.
Because of its small z direction thickness, e.g., typically about 10 to 100 angstroms, MTJ freelayers require gentle etching solutions, compared to typical microelectronic back end of the line (BEOL) films that are typically greater about 100 times the thickness of MRAM layers, to minimize lateral etching and to maximize etching selectivity with respect to the thin tunneling barrier layers, e.g., MgO or AlOx.
Etching solutions have previously been proposed for alumina-based tunnel barrier layers. One such solution is disclosed in U.S. Pat. No. 7,252,774 to O'Sullivan et al., which provides an etchant solution that, includes a surfactant inhibitor, e.g., an alkyl sulfonate inhibitor, and at least one weakly adsorbing acid, e.g., perfluoroalkane sulfonic acids. Because the IEP of bulk AlOx is about 9, the zero charge of the alumina-based tunnel barrier is significantly greater than the pH of the etchant solution. The negatively charged SO3− groups of the sulfonate surfactant inhibitor adsorb onto the positively charged sites of the tunnel barrier surface, thereby protecting the tunnel barrier from dissolution or significantly slowing such dissolution by the acid.
MgO has an isoelectric point (IEP) of about 13, which is significantly higher than that for AlOx (IEP is about 9). As such, MgO has a much higher dissolution rate in acidic solutions relative to AlOx. Thus, the gain in stability for MgO tunneling barriers in the presence of the alkyl sulfonate inhibitors has been found to be much less than that observed for AlOx tunneling barriers, i.e., about 100 times less stable. Thus, the prior art solution for etching the freelayers disposed on alumina are not adequate at providing sufficient protection of magnesium oxide based tunneling barriers.
Accordingly, there is a need for etchant solutions for selectively removing the freelayer from MgO tunneling barriers.